Are we sure about tetrapod evolution? Tiktaalik takes the Challenge!

Before learning about extant herps, we had to learn about how herps became herps and the evolutionary history of tetrapods. One of the most well known stem tetrapods that we also discussed in lecture was Tiktaalik from the mid to late Devonian (393 - 359 mya). Research compiled by Shubin et al. is attempting to shine light on how we understand tetrapod evolution because of research that had not previously been focused on. When we think tetrapods we think 4 limbs, but even with that said it is important to understand how the 4 limbs became what they are especially since we see evidence in the fossil record of the pelvic and pectoral girdle and appendages having different origins and evolutionary history. Previously because of the fossils being poorly preserved or not found, the pelvic girdle has been ignored with most of the focus on the pectoral girdle. Because of this the "front wheel drive hypothesis was created explaining that the closest tetrapod relatives were more emphasized on pectoral support for locomotion and that locomotion and support from the pelvic girdle is actually a tetrapod innovation. Most of the fossils of stem tetrapods (tetrapomorphs) examined showed a much larger pectoral girdle with a very small pelvic girdle, like we see in fish. Then once we see what we call tetrapods the pelvic girdle is much more robust, larger and has extra anatomical features that help with muscloskeletal support for both posture and locomotion. Although, this new research includes findings from 5 Tiktaalik specimens discovered with pelvic girdle and appendages that were decent enough for examination. In so Shubin et al. show that the pelvic girdle of Tiktaalik which because of it being a finned tetrapomorpha should have a much smaller pelvic girdle, actually shows ratios similar to those seen in tetrapods. The pelvic girdle also showed other interesting features such as a deep acetabulum that would allow more lateral orientation of the appendages and a very broad iliac blade that made it to the level at least of the vertebral column. It does not have a sacral rib which helps in support, but the other details are show traits that were not noted before. Overall this research shows that from analyzing the remains of Tiktaalik, "pelvic propelled locomotion in stem tetrapods actually began in the finned taxa before enhanced in more derived digit forms."
This research is not only interesting but it is a game changer. This could show a difference in how we understand tetrapod  evolution and the transition between water and land and overall the evolution of herps. This research was compiled and completed by Neil H. Shubin, Edward B. Daeschler and Farish A. Jenkins, Jr. and presented and published for the PNAS.


Kimmy Profitko
Article Review
(Link) http://www.pnas.org/content/111/3/893.full.pdf